1. Field of the Invention
The present invention relates to an electrically conductive roll used in an electronic photo-copying machine, for example.
2. Related Art Statement
An electronic photo-copying machine produces a copy of an original image by forming a latent image of the original on a circumferential surface of a photosensitive drum, applying toner to the drum surface thereby forming a toner image, and transferring the toner image onto a copy sheet. For forming a latent image on the drum surface, it is required to electrically charge the drum surface in advance, and expose the charged drum surface to an original image through an optical system, thereby eliminating the electric charge from the exposed portions on the drum surface.
There have been known two types of electrical charging of the drum surface; corona charging and "contact" charging. In the corona charging method, the drum surface is charged by corona discharge produced from a corona discharging device. The corona discharging needs a high-voltage power supply (generally, 5 to 10 KV), therefore complete safety care must be taken. In addition, this charging method suffers from a disadvantage that, upon discharging, ozone is generated which is harmful to human beings. For this reason, attention has recently been directed to the contact charging method in which the drum surface is charged by frictional contact thereof with an electrically conductive roll.
FIG. 1 shows an electronic photo-copying machine employing the above-identified contact charging method. This copying machine produces a copy of an original 8, as follows: When a photosensitive drum 1 is rotated about an axis 1a in a direction indicated at arrow in the figure, an electrically conductive roll or charging roll 2 is rotated on the outer circumferential surface of the drum 1 in the same (or reverse) direction as that of the drum 1, in such a manner that friction occurs between the drum 1 and the roll 2 with the roll 2 elastically or resiliently being deformed. Reference numeral 3 denotes an exposing device having a slit through which the original image 8 is incident to the drum surface so as to form a latent image corresponding to the original 8. Numeral 4 designates a developing device which applies toner to the latent image thereby forming a toner image. Numeral 6 denotes sheet feeding rolls which feed a copy sheet 11 toward the drum surface, and a transferring device 5 transfers the toner image onto the copy sheet 11. Numeral 7 designates fixing rolls which fix the thus reproduced image on the sheet 11 when the sheet 11 passes therethrough. Thus, a copy of the original is produced.
A cleaning device 9 serves for removing the toner residue, from the drum surface. In addition, an eraser lamp 10 irradiates the entire surface of the drum surface so that the drum surface becomes zero potential. Numeral 12 designates a power supply which applies a 1 to 3 KV electric voltage to the charging roll 2.
An electronic photo-copying machine or the like employs a lot of electrically conductive rolls (hereinafter, referred to as the "conductive rolls"), such as charging roll, developing roll, transferring roll, cleaning roll, etc. These conductive rolls are required to have an electric resistivity (i.e., specific resistance) in a range of 10.sup.1 to 10.sup.12 .OMEGA..multidot.cm. As shown in FIG. 2, a conductive roll consists of a metal shaft (metal core) 21, and an electrically conductive resilient layer 22 formed on the outer circumferential surface of the core 21. Conventionally, the resilient layer 22 is formed of a mixture composition including synthetic rubber such as silicone rubber, and electrically conductive powder or fiber such as carbon black, metal powder, or carbon fiber.
Out of the above-indicated various sorts of conductive rolls, the charging roll in particular is required to have an electric resistance in a semi-conductive range of 10.sup.5 to 10.sup.7 .OMEGA..
However, regarding the charging roll of FIG. 2, the conductivity of the resilient layer 22 is created by contact between the conductive particles dispersed in the synthetic rubber. Generally, such particle contact is not uniform in the rubber. This problem particularly occurs with respect to the above-indicated semi-conductive range. Thus, it is difficult to obtain a charging roll having a desirable electric resistance in the range of 10.sup.5 to 10.sup.7 .OMEGA.. This problem results in failing to reproduce an excellent copy of an original image.
For solving the above-identified problem, it has been proposed to provide a resistance adjusting layer 23 on the outer surface of an electrically conductive resilient layer 22 of a charging roll, as shown in FIG. 3. However, common synthetic resins (e.g., polyethylene resin, polyester resin, epoxy resin) and common synthetic rubbers (e.g., ethylene-propylene rubber, styrene-butadiene rubber, chlorinated polyethylene rubber) each are an insulating material having an electric resistivity of more than 10.sup.12 .OMEGA..cm. In order that the resistance adjusting layer 23 is formed of one of these resins and rubbers so that the roll enjoys a suitable electric resistance of 10.sup.5 to 10.sup.7 .OMEGA., it is required that the thickness of the adjusting layer 23 be smaller than 1 .mu.m. However, such a thin layer 23 has no durability in service.
As is apparent from the foregoing, the conventional conductive rolls, in particular charging roll, lack resistance uniformity and serviceability, and therefore are not satisfactory.
In this situation, the inventors of the present application and another person had filed a Japanese patent application, which was laid open under Publication No. 1-142569 on Jun. 5, 1989. For overcoming the above-identified problems, we proposed to form a resistance adjusting layer of epichlorohydrin-ethylene oxide copolymer rubber (abbreviated to "CHC"). This adjusting layer enjoys uniform resistance, and sufficient adjustability to a suitable thickness for actual use or service. However, depending upon a rubber composition for the conductive resilient layer positioned under the resistance adjusting layer, a softening agent, such as oil, possibly contained in the composition may permeate, through the adjusting layer, to the outer surface of the conductive roll or charging roll. In this case, the oil may further be transferred to the outer surface of the photosensitive drum which is disposed in pressed contact with the charging roll. If the oil is spread on the drum surface, some of the toner representing an original image is adhered to the drum surface and is not transferred onto a copy sheet. Consequently, an image reproduced on the copy sheet lacks some part of the original image.
The photosensitive drum suffers from another problem that a resin film or coating provided on the drum surface is deteriorated because of permeation thereinto of the oil. Furthermore, since portions of the drum surface to which the oil is adhered cannot be electrically charged, those portions act as if they are charged and subsequently discharged by exposure to image light. In the case where discharged portions on the drum surface represent a positive image reproduced on a copy sheet, the oil-covered portions act as representing some of an image to be reproduced. It is known that, when oil is adhered to the roll surface, it often assumes a line extending in the axial direction of the roll, and consequently a black line appears as a part of a reproduced image on a copy sheet.